Combine Geometry
----------------
Doing the union operation on two geometries is one of the most expensive operations you can possibly do; while it is reasonable for small numbers (say 5-10) when you start to get up to hundreds of geometries the cost can be measured in minutes.
* Using GeometryCollection union()
In JTS 1.9 there is a new union() method that will "do the right thing"::
static Geometry combineIntoOneGeometry( Collection geometryCollection ){
GeometryFactory factory = FactoryFinder.getGeometryFactory( null );
// note the following geometry collection may be invalid (say with overlapping polygons)
GeometryCollection geometryCollection =
(GeometryCollection) factory.buildGeometry( geometryCollection );
return geometryCollection.union();
}
* Using buffer( 0 )
You can get the same effect in JTS 1.8 using buffer(0)::
GeometryFactory factory = FactoryFinder.getGeometryFactory( null );
// note the following geometry collection may be invalid (say with overlapping polygons)
GeometryCollection geometryCollection =
(GeometryCollection) factory.buildGeometry( geometryCollection );
Geometry union = geometryCollection.buffer(0);
* Using union( geometry )
Using JTS versions prior to 1.9 you will need to combine the geometries one by one using geometry.union( geometry )::
static Geometry combineIntoOneGeometry( Collection geometryCollection ){
Geometry all = null;
for( Iterator i = geometryCollection.iterator(); i.hasNext(); ){
Geometry geometry = i.next();
if( geometry == null ) continue;
if( all == null ){
all = geometry;
}
else {
all = all.union( geometry );
}
}
return all;
}
The above code is pretty much too simple to live; the correct way to do things is to break up your data into regions, union all the geometries in
one region together; and then combine these at the end into one big geometry (this is the approach used above by the union() method).